1. Technical Field
The present invention relates to rotors for rotating electric machines that are used in, for example, passenger cars and trucks as electric motors and electric generators.
2. Description of Related Art
Conventionally, there is known a method of fixing a cooling fan to an axial end face of a magnetic pole core of a rotor by projection welding with a spherical projection formed on one surface of the cooling fan (see, for example, Japanese Unexamined Patent Application Publication No. 2000-92787). With this method, the parts count of the rotor can be reduced. Moreover, since it is unnecessary to perform a bolt-fastening process for fixing the cooling fan to the magnetic pole core, the productivity of the rotor can be improved.
On the other hand, the electrical load capacities of motor vehicles vary depending on the design specifications of the vehicles. Therefore, considering the use of automotive alternators in various motor vehicles, it is necessary to increase variation in the outputs of the alternators. Generally, the outputs of the alternators are proportional to D2L, where D and L are respectively the inner diameter and axial length of a stator core of the alternator. Moreover, varying D is preferable to varying L in terms of increasing variation in the outputs of the alternators while minimizing dimensional change in the alternators.
However, when D is increased for increasing the output of an automotive alternator, the outer diameter of a rotor of the alternator is accordingly increased; the rotor is rotatably disposed radially inside the stator core with a predetermined gap formed between the radially outer periphery of the rotor and the radially inner periphery of the stator core. In this case, to increase the strength of the cooling fan against the centrifugal force, it is necessary to shift the welding position radially outward. Consequently, it becomes necessary to replace the welding electrodes according to the change in the welding position, thereby complicating the manufacturing process and increasing the manufacturing cost.
In addition, it may be possible to eliminate the need of replacing the welding electrodes by employing a special welding apparatus. However, in this case, it would be still difficult to reduce the manufacturing cost due to the maintenance necessary for the special welding apparatus.
The above problem can be solved by applying laser welding instead of projection welding. More specifically, laser welding does not require welding electrodes. Therefore, in the case of applying laser welding instead of projection welding, it is unnecessary to replace welding electrodes according to change in the welding position. Consequently, it is possible to simplify the manufacturing process and reduce the manufacturing cost.
On the other hand, in operation of the alternator, the cooling fan is subjected to water which flows to the cooling fan along with the cooling air. Therefore, to prevent the cooling fan from being rusted by the water, the cooling fan is generally made of a steel plate which is plated with, for example, zinc. However, in this case, during the laser welding of the cooling fan to the magnetic pole core, metal gas will be produced by the melting of the plating metal (e.g., zinc). Consequently, due to the metal gas, it becomes easy for voids to be formed in the molten portions of the cooling fan and the magnetic pole core, thereby lowering the welding strength and thus the reliability of the rotor.